Examples of electrochemical elements include aluminum electrolytic capacitors, conductive polymer aluminum solid electrolytic capacitors, conductive polymer hybrid aluminum electrolytic capacitors, electric double-layer capacitors, lithium-ion capacitors, lithium-ion secondary batteries, and lithium primary batteries.
A separator in an electrochemical element serves to isolate both electrodes from each other and to hold an electrolytic solution. To isolate both foil electrodes, the separator is required to combine high shielding properties with low resistance. The material of the separator is required to have electrical insulation. To hold various types of electrolytic solution, the material is required to have hydrophilicity and lipophilicity.
Cellulose has these properties. Cellulose paper has long been used as a separator for an electrochemical element. Among cellulose fibers, beatable regenerated cellulose fibers are characterized by generating fibrils having high stiffness and a small fiber diameter when highly beaten. It is known that the use of beaten regenerated cellulose fibers can produce a microporous, highly dense separator. In recent years, thus, separators formed of beatable regenerated cellulose fibers have often been reported.
PTL 1 discloses a method in which beatable regenerated cellulose fibers are used in order to improve the denseness and the impedance characteristics of a separator. The separator produced by the use of the beaten regenerated cellulose fibers is formed of highly dense, microporous paper. In the case of producing aluminum electrolytic capacitors using the separators, the percentage of defective products due to short circuits is reduced, and the impedance characteristics are improved.
However, when a separator composed of 100% by mass of beatable regenerated cellulose fibers is used as described in PTL 1, because the separator is low in tensile strength and tear strength, the separator is sometimes broken in a process for producing an aluminum electrolytic capacitor. The separator has low strength and thus has low resistance to portions, such as tab portions and foil burrs, of the aluminum electrolytic capacitor that applies stress thereto, causing breakage of the separator to cause a short circuit, in some cases.
PTL 1 also discloses that an abaca pulp, a sisal hemp pulp, and so forth are mixed with the beatable regenerated cellulose fibers. PTL 2 discloses a separator for an electric double-layer capacitor, the separator being formed of solvent spun rayon, which is categorized into beatable regenerated cellulose fibers, and a sisal pulp.
When beatable regenerated cellulose fibers are mixed with the abaca pulp and the sisal hemp pulp, the tensile strength and the tear strength are improved; however, the formation of the separator is disadvantageously degraded because the abaca pulp and the sisal hemp pulp that are little beaten and that have a high CSF value have long fiber lengths. Long fiber lengths make it more difficult to uniformly disperse the fibers in water, causing difficulty in forming a uniform paper sheet during papermaking. To improve the formation, it is necessary to promote the beating of the abaca pulp and the sisal hemp pulp to shorten the fiber lengths. However, when the abaca pulp and the sisal hemp pulp are beaten, the impedance characteristics are markedly degraded; thus, substantially no beating has been performed.
PTL 3 discloses a separator for an electrochemical element, the separator being composed of cellulose fibers obtained by specifying the freeness, the length-weighted average fiber length, and the pattern of the fiber length distribution histogram of solvent spun cellulose fibers.
However, the percentage of the solvent spun rayon is preferably 80% or more in view of the holding of an electrolytic solution. The percentage of the solvent spun rayon contained is high, thereby resulting in the low-strength separator. The separator is inevitably broken in a process for producing an electrochemical element.
PTL 3 also discloses a cylindrical separator containing 10% to 25% linters having a modified freeness of 270 ml (about 30 ml in terms of CSF) and a cylindrical separator containing 25% abaca fibers having a modified freeness of 820 ml (about 730 ml in terms of CSF). Highly beaten linters have poor impedance characteristics, and the fibers are curly and thus have insufficient strength. The abaca fibers are in an unbeaten state. Thus, a highly dense separator cannot be produced. In the case of a separator produced with a cylinder paper machine, the formation of pinholes attributed to the wire pattern of a cylinder during papermaking is inevitable. Thus, the technique is not appropriate for the production of a highly dense separator.
PTL 4 discloses a separator for a capacitor, the separator being formed of solvent spun cellulose fibers and solvent spun cellulose short fibers having specified freeness and length-weighted average fiber length. The solvent spun cellulose short fibers used here are not beaten and thus are straight fibers having a fiber length of 3 to 5 mm. A higher content of unbeaten solvent spun cellulose fibers results in decreases in the strength and denseness of paper because the entanglement of the fibers does not occur. In PTL 4, the solvent spun cellulose short fibers are used to improve the transfer state of a wet web to felt. Attempts to improve the strength and the denseness by beating are highly likely to degrade the transfer state of the wet web.
PTL 5 discloses a separator for a lithium-ion secondary battery, the separator containing 10% to 30% by mass fibrillated solvent spun cellulose fibers, 40% to 50% by mass oriented, crystallized polyester short fibers having an average fiber diameter of 2.0 to 3.5 μm, and 30% to 40% by mass unstretched polyester short fibers, serving as a binder, having an average fiber diameter of 5.0 μm or less.
Hydrogen bonds, which are key factors for the strength of paper, are formed between cellulose fibers, but are not formed between the solvent spun cellulose and the oriented, crystallized polyester short fibers. Thus, for the purpose of holding the form of paper, the thermally fusible unstretched polyester short fibers for a binder are contained. The oriented, crystallized polyester short fibers and the unstretched polyester short fibers for a binder are straight fibers. The high contents of these synthetic fibers make it difficult to increase the denseness of the separator having the composition described in PTL 5. If the conditions of hot calender treatment are adjusted to promote the fusion of the binder fibers, the denseness can be increased. However, the binder fibers are transformed into films; thus, the impedance characteristics are markedly degraded. Cellulose is a material having a decomposition temperature of about 260° C., no softening point, and thus good heat resistance. When a common synthetic fibers are used, the degradation of the heat resistance of the separator is inevitable.